For example, as disclosed in Patent Literature 1 described below, there is publicly known an electric power tool that adopts a brushless motor (brushless DC motor) as a drive source. The brushless motor displays high efficiency when compared with a commutator motor, and when used in a cordless-type electric power tool using a rechargeable battery (battery pack), operation time of the electric power tool per charge is increased when compared with use of the commutator motor. Also, the electric power tool using a brushless motor is mounted with a drive control circuit for the motor, which includes a microcomputer and the like, and therefore this type of electric power tool uses electronic control to facilitate advanced rotational control of the motor.
As well known, the brushless motor includes: a magnet rotor provided with a permanent magnet; a stator provided with multiple-phase stator coils such as three-phase coils; a plurality of magnetic sensors respectively including Hall ICs for detecting a rotor position by detecting magnetic force of the permanent magnet of the rotor; and an inverter circuit board that uses a plurality of semiconductor switching elements (for example, 6 elements) such as MOSFETs (insulating gate field-effect transistors), IGBTs (insulating gate bipolar transistor), or the like to switch DC voltage, convert the DC voltage into a three-phase AC voltage, and thereby apply current to the respective phase stator coils. The plurality of magnetic sensors correspond to the multiple-phase stator coils, and the brushless motor is configured to set a current application timing for each of the multiple-phase stator coils on the basis of a rotor position detection result by each of the magnetic sensors.
In the brushless motor, a large current is applied to each of the semiconductor switching elements (power transistors) mounted on the inverter circuit board, so that a power loss is increased to thereby increase a calorific value of the semiconductor switching element. When the calorific value of the semiconductor switching element is increased, the inverter circuit board, the semiconductor switching elements mounted on the inverter circuit board, and other circuit elements are raised to high temperature to receive thermal damage, which causes deterioration or breakage.
For this reason, as disclosed in Patent Literature 1, an approach has been considered, in which, in order to forcibly cool the semiconductor switching elements mounted on the inverter circuit board housed in a body housing portion of the electric power tool, the semiconductor switching elements on the inverter circuit board are disposed in proximity to an outer peripheral portion of a cooling fan attached to a motor rotation shaft to thereby actively radiate heat produced in the semiconductor switching elements.    [Patent Literature 1] Unexamined Japanese Patent Application KOKAI Publication No. 2006-297532
However, when the brushless motor adopting the power transistors including radiation fins as the semiconductor switching elements as described above is adopted for an electric power tool, the inverter circuit board mounted with the semiconductor switching elements must be installed in proximity to the motor main body in the body housing portion housing the motor portion. For this reason, a problem of increasing the overall length of the body housing portion arises, or a problem of increasing a circumferential dimension of the motor rotation shaft in the body housing portion arises. As a result, it is difficult to improve the overall axial length or radial dimension in the body housing portion of the electric power tool so as to achieve reduced and compact one, and operation or workability characteristics of the tool are adversely affected.
In particular, when a high-output power transistor such as a MOSFET is adopted as the semiconductor switching element on the inverter circuit as disclosed in Patent Literature 1, it is necessary to improve a heat radiation effect by independently mounting the power transistor in a perpendicular orientation to the inverter circuit board using an insertion mounting technique on a lead line of the power transistor, and by providing a heat radiation plate (heat sink) on a drain electrode side, and consequently there arises a problem that the overall axial length of the body housing portion is increased. For this reason, it is required to reduce the overall axial length of the body housing portion in view of the operation or workability characteristics of the tool.
It is therefore an object of the present invention to provide an electric power tool that has a low overall length of a body housing portion and adopts a brushless motor.
Another object of the present invention is to provide an electric power tool that has a high cooling effect on a semiconductor power transistor on an inverter circuit board disposed in a body housing portion.